Mehdi Takabi
Nowadays, construction projects are usually so complex that project schedulers must inevitably use various calendars when developing the project schedule in the associated software like Primavera (P6).
Although Primavera (P6) brings an excellent tool for the project schedulers to define multiple calendars, the schedulers may encounter unexpected negative total float. There are several possible causes for having negative floats, such as:
- Constraints on the activities
- A ‘Must Finish By’ constraint on the project
- External relationship
- Calendars
Most of the schedulers are knowledgeable about the above root causes of the negative floats since they are relatively easy, except calendars. In this article, we will demonstrate how calendars could lead to negative total float values in a simple scenario.
The below picture shows the defined activities for typical foundation works.
As this schedule shows, the assigned calendar for the finish milestone of ‘Inspection of Foundation’ is the same as its predecessor calendar (Concrete Pouring activity).
Now, imagine the inspection team announcing that they can work only on Fridays every week. To model it, you need to define a unique calendar with only workdays on Fridays every week.
Once you assign this calendar to the milestone in the schedule and re-schedule the project, you face negative total floats on all other tasks in the network path that lead to this milestone.
Since the milestone has been defined as a Finish Milestone, the late dates of the predecessor activities are earlier than the early dates, causing negative floats. This applies to both the Finish Float and the Start Float options in the Scheduling Options window.
This problem is solved by defining the milestone as Start Milestone. In this case, the late dates of the predecessors are later than the earlier dates, as expected, and the negative float disappears.
Although there is no explanation in the Oracle documentation regarding this problem, Oracle seems aware of the issue. Because of that, a particular item in the warnings section of the ‘Schedule Log’ is dedicated to the Finish Milestones with different calendars than its predecessors.
Conclusion
As explained, without constraints and external relationships, we might still have negative floats just because of the calendars. Therefore, schedulers are recommended to check the schedule log warning section to determine the cause of the negative float.
